While the service had a scheduled downtime for maintenance last night, they overshot their window by several hours, even extending into today after founder Ev Williams wrote a post on the Twitter blog apologizing for the downtime and declaring the service back on line.

To make matters worse, they had just written a post yesterday praising their infrastructure and its high scalability – a scalability which many users questioned after the service crashed under the weight of usage during MacWorld.

But it’s not just the users who are struggling to cope with Twitter’s downtime. Twitter’s service have become a valuable part of many other start-ups and serves a complimentary role to some large services.

Tweetmeme and Politweets are two start-ups that rely completely on Twitter for their services to work. FriendFeed meanwhile imports all users’ tweets into its activity streams. Likewise, Yahoo’s MyBlogLog also has Twitter integration as a main component of users’ profiles. Then of course there is Facebook where many users tie their status updates directly to their Twitter updates.

The point of Twitter’s infrastructure post was to alert people that they were ready for this weekend’s upcoming Super Bowl, and the massive amount of tweets it may bring. One can only hope that this latest round of maintenance gets the system ready for an upcoming schedule that includes not only Sunday’s game, but Super Tuesday, and the SXSW festival in March – which coincidentally is where Twitter first took off among early adopters last year.

Twitter is becoming an increasingly important application for the Internet at large and if it can’t be relied upon, it causes issues for more than just disgruntled users.